1. Field of the Invention
This invention relates generally to a process for the bromination of polystyrenic homo- and co-polymers using bromine as the reaction medium and bromine chloride as the bromination reagent and in which the degree of bromination can be controlled by adjusting the molar ratio of the bromination reagent to polystyrene.
2. Description of the Prior Art
In the past, numerous processes have been employed for brominating polystyrene. None of these processes has been found to be entirely successful; all have been found to have disadvantages.
Previously known polystyrene bromination processes have utilized reaction media such as a chlorohydrocarbon. See, e.g., Naarman et al., U.S. Pat. Nos. 4,074,032; Naarman et al, 4,143,221; Barda et al., 4,352,909; Diebel et al., Fed. Rep. of Germany Pat. No. 2800012. None of these references disclosed the use of bromine as the reaction medium.
At the same time, bromination of polystyrene by the prior art process has frequently been achieved in the presence of a catalyst, for example, aluminum trichloride or ferric chloride, having activities ill-suited to controlled bromination reactions. In Diebel et al., U.S. Pat. No. 4,200,703, the use of aluminum trichloride and ferric chloride as suitable catalysts for brominating polystyrene is disclosed, but the process requires that the catalyst be moderated by the addition of a nucleophilic substance such as water to avoid crosslinking of the polymer. See, e.g., Barda, U.S. Pat. No. 4,223,169 (disclosing the use of antimony chloride catalyst and moderation of the catalyst by the addition of water); Diebel et al., U.S. Pat. No. 4,200,703. See also, Barda, U.S. Pat. No. 4,352,909 (disclosing antimony chloride catalyst in a chlorohydrocarbon reaction medium without a moderating agent); Jalics, U.S. Pat. No. 4,028,486 and Kainmuller et al., Canadian Pat. No. 1,124,947.
Bromination of polystyrene according to prior art methods requires, in many instances, either pre-treatment of the polystyrene by hydrogenation to eliminate olefinic double bonds (Naarman et al., U.S. Pat. No. 4,074,032) or cationic polymerization of styrene in situ using the same catalyst intended as the bromination catalyst (Naarman et al., U.S. Pat. Nos. 4,143,221; Lindenschmidt et al., 4,360,455).
In addition to the foregoing prior art brominations of polystyrene, numerous processes have been employed in the past for brominating other aromatic compounds, such as diphenyl ether, phenol, toluene, xylene and napthyl ethers in a bromine reaction medium. None of these processes has been taught to be effective in the bromination of polystyrene.
Stepniczka, U.S. Pat. No. 3,965,197 discloses a process for the complete bromination of non-fused ring aromatic compounds, such as benzene, toluene, xylene, phenol and diphenyl ether. The disclosed process uses liquid bromine as the reaction solvent as well as the bromination reagent. A catalyst such as aluminum, iron, or their corresponding halides is used to effect the bromination at a temperature of about 10.degree. C. to ambient. Similar processes are disclosed in British Patent Specification No. 1,411,524 and in German Pat. No. 2,400,455.
Bay et al, U.S. Pat. No. 4,546,139, issued Oct. 8, 1985, is a composition patent which describes the applications of polybromodinaphthyl ethers. It teaches a process for the bromination of naphthyl ethers using liquid bromine as the reaction solvent and bromination reagent. The procedure uses a Lewis acid catalyst such as aluminum trichloride, with a reaction temperature of 15.degree. C. in order to effect the bromination. The products which result contain approximately 75% bromine.
In Garman et al, U.S. Pat. No. 4,287,373 a process for the bromination of phenol and diphenyl ether is described using liquid bromine as the reaction medium. The process, which uses a Lewis acid catalyst such as aluminum or iron halides at elevated temperatures of at least 35.degree. C. up to 55.degree. C., affords an essentially pure prebrominated product.
The Stepniczka, Bay, and Garman processes are not effective in controlling the bromination level on aromatic rings of polystyrene, or poly-(p-methylstyrene) or certain other styrenic-based polymers. Moreover, the processes disclosed in those patents call for the complete bromination of the identified aromatic compounds. Controlling the reaction to achieve a desired partial level of bromination is not disclosed and is, indeed, not possible by those processes. In addition, because the bromination reagent, bromine, in the Stepniczka, Bay, and Garman processes is relatively weak, a corresponding strong catalyst, such as aluminum trichloride, must be employed. In the case of polystyrenes and other styrenic polymers such a catalyst increases the amount of coincident polymer degradation and may promote polymer crosslinking, resulting in a less desirable product with decreased thermal stability and increased processing difficulties.
Brominated polystyrenic polymers prepared by this invention can be used to impart flame retardancy to many normally flammable materials such as thermoplastics and thermoset resins.
Accordingly, it is the primary object of the present invention to provide a method for brominating polystyrene to produce a thermally stable, flame-retardant additive.
Another object is to provide a method for brominating polystyrene in which the degree of bromination is controlled by adjusting the molar ratio of the bromination reagent to polystyrene.